Print a warning if southbridge is not known

Corresponding to flashrom svn r61 and coreboot v2 svn r2390.
diff --git a/README b/README
index 32c401a..e745608 100644
--- a/README
+++ b/README
@@ -77,6 +77,6 @@
 -----------
 
 DISK on Chip support is currently disabled since it is considered unstable. 
-Change CFLAGS in the Makefile to enable it.
+Change CFLAGS in the Makefile to enable it: Remove -DDISABLE_DOC from CFLAGS.